KMP与sunday的比较

有关sunday算法的讲解：http://blog.csdn.net/caianye/article/details/6096610

以下转载请注明出处   by  CrazyAC

1.求模式串出现在文本串的第一个位置

情况1：

   char src[]="jfkdsahdiojdaigfgthlipjgffg";
   char des[]="gffg";

KMP：（匹配了49次）

+ View Code?

#include <iostream>

using namespace std;

 

char src[]="jfkdsahdiojdaigfgthlipjgffg";

char des[]="gffg";

int next[10];

int n, m;

 

 

void getNext() {

    int i, j;

    i = 0;

    j = -1;

    n = strlen( src );

    m = strlen( des );

    next[0] = -1;

    while( i < m ) {

        if( j == -1 || des[i] == des[j]) {

            i ++;

            j ++;

            next[i] = j;

        } else {

            j = next[j];

        }

    }

}

 

 

void solve() {

    int i, j;

    i = j = 0;

    int cnt = 0;

    while( i < n && j < m ) {

        cnt ++;

        if( j == -1 || src[i] == des[j] ) {

            i ++;

            j ++;

        } else {

            j = next[j];

        }

    }

    if( j == m ) printf( "%d\n", i-j+1 );

    else printf( "-1\n" );

    printf( "cnt = %d\n", cnt );

}

 

 

int main() {

//    freopen( "c:/aaa.txt", "r", stdin );

    getNext();

    solve();

    return 0;

}

sunday:  (匹配了10次）

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#include <iostream>

using namespace std;

 

 

char src[]="jfkdsahdiojdaigfgthlipjgffg";

char des[]="gffg";

int next[26], cnt;

 

 

int sunday() {

    int i, j, pos, sum;

    int len_s = strlen( src );

    int len_d = strlen( des );

 

    for( i=0; i<26; ++i ) next[i] = len_d + 1;

    for( i=0; i<len_d; ++i ) next[des[i]-'a'] = len_d - i;

 

    pos = sum = 0;

    while( pos < (len_s-len_d+1) ) {

        for( i=0; i<len_d; ++i ) {

            ++ cnt;

            if( src[pos+i] != des[i] ) {

                pos += next[ src[pos+len_d] - 'a'];

                break;

            }

        }

        if( i == len_d ) return pos;

    }

    return -1;

}

 

 

int main() {

    cnt = 0;

    printf( "%d\n", sunday() );

    printf( "cnt = %d\n", cnt );

    return 0;

}

 可见sunday的优越性。但是如果测试数据为

char src[]="aaaaaaaaaaaaaaaaaaaaba";
char des[]="aaaaaaaba";

KMP匹配了35次，sunday匹配了105次！、

2.求模式串在文本串中出现的次数。hdoj 1686

sunday超时

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#include <iostream>

#include <cstring>

#include <cstdio>

using namespace std;

 

 

char src[1000005], des[10005], next[26];

 

int sunday() {

    int i, j, pos, sum;

    int len_s = strlen( src );

    int len_d = strlen( des );

 

    for( i=0; i<26; ++i ) next[i] = len_d + 1;

    for( i=0; i<len_d; ++i ) next[des[i]-'A'] = len_d - i;

 

    pos = sum = 0;

    while( pos < (len_s-len_d+1) ) {

        for( i=0; i<len_d; ++i ) {

            if( src[pos+i] != des[i] ) {

                pos += next[ src[pos+len_d] - 'A'];

                break;

            }

        }

        if( i == len_d ) {

            ++sum;

            if( pos + len_d == len_s ) break;

            pos += next[ src[pos+len_d] - 'A' ];

        }

    }

    return sum;

}

 

 

int main() {

//  freopen( "c:/aaa.txt", "r", stdin);

    int T;

    scanf( "%d", &T );

    while( T-- ) {

        scanf( "%s %s", des, src );

        printf( "%d\n", sunday() );

    }

    return 0;

}

KMP:  93MS

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#include <iostream>

#include <cstdio>

#include <cstring>

using namespace std;

 

 

char src[1000010], des[10010];

int next[10010];

int len_d, len_s;

 

void getNext() {

    int i, j;

    i = 0;

    j = -1;

    next[0] = -1;

    while( i<len_d ) {

        if( j == -1 || des[i] == des[j] ) {

            i ++;

            j ++;

            next[i] = j;

        } else {

            j = next[j];

        }

    }

}

 

 

void solve() {

    int i, j, sum = 0;

    i = j = 0;

    while( i < len_s ) {

        if( j == -1 || src[i] == des[j] ) {

            i ++;

            j ++;

        } else {

            j = next[j];

        }

        if( j == len_d ) {

            ++ sum;

            j = next[j];

        }

    }

    printf( "%d\n", sum );

}

 

 

int main() {

 //   freopen( "c:/aaa.txt", "r", stdin);

    int T;

    scanf( "%d", &T );

    while( T-- ) {

        scanf( "%s %s", des, src );

        len_d = strlen( des );

        len_s = strlen( src );

        getNext();

        solve();

    }

    return 0;

}

综上所述，在信息学竞赛中sunday算法的优势得不到体现，那是因为ACM比赛对时间的重视，所以测试数据会卡你时间，而这些卡你时间的测试数据又往往把sunday给卡住了

就像上面这组数据

char src[]="aaaaaaaaaaaaaaaaaaaaba";
char des[]="aaaaaaaba";

根据sunday的原理，当一直匹配到b时，

 aaaaaaaaaaaaaaaaaaaaba

 aaaaaaaba

发生不匹配，根据sunday中，移动步长=匹配串中最右端的该字符到末尾的距离+1 ，而最右端是‘a'，’a'这个字符到末尾的距离为0，所以步长为1，

又要从第二个'a'开始匹配

 aaaaaaaaaaaaaaaaaaaaba

   aaaaaaaba

如此和暴力就相差无几了。

对KMP来说，当到b不匹配时，i = j = 7，然后，j = next[j] = 6，所以，下次只是对src[i]与des[j]进行比较

 aaaaaaaaaaaaaaaaaaaaba

   aaaaaaaba

前面那一串红色的a的比较就可以省掉了，而sunday是没有省掉的。